The present invention relates to a web transport system, and more particularly to a drive system for the takeup and supply spools of a web transport. A wide variety of web transport systems are available for different applications, such as magnetic tape transports, transports for cinematographic film, and the like. These systems generally require careful handling of the web, with more or less rigorous requirements depending on the given applications.
A principal design criterion for the tape transport system of a magnetic tape cleaner, for example, is that the tape travel at a desired speed profile and tension profile. Among the primary components of a typical transport system are two spools for supply and takeup of magnetic tape, which store the magnetic tape and feed the tape to and from the cleaning stations (deck components). The rotation of these spools is regulated by a drive system, usually involving one or more motors and electronics for delivering power to the motors.
It is generally desirable that magnetic tape maintain the desired tension profile during winding and unwinding of the spools. The drive system is advantageously designed so that the tape will not be unduly stressed at one or the other of the spools under operating conditions. In the particular case in which the rotation of each spool is controlled by a separate motor (allowing more independent control), the torque delivered by each motor should be correlated with the winding diameter of the corresponding spool. Thus, to maintain an even tension, the torque delivered by a motor should increase as the winding diameter increases and motor speed decreases, assuming that the tape is transported at a constant speed.
In prior art tape transport systems, AC motors have typically been used to drive the takeup and supply spools. These motors are characterized in that they produce a torque which matches the load to the motor. Torque will thus tend to decrease with motor speed, which will create an undesirable tendency of tape tension to fall as a spool becomes fuller. More recently, direct current motors have been found to have superior characteristics in this regard. Where the stator comprises a permanent magnet (in a "permanent magnet motor"), the torque produced by the motor is reasonably proportional to the current to the motor, providing increased control of tape tension. The design of transport systems incorporating DC motors, however, requires that the controlling electronics be adapted to changes in winding diameter and to the need for smooth startup and braking of spool rotation.
Accordingly, it is one object of the invention to provide a control system for the takeup and supply spools of a web transport. A related object is that such a system allow the transport of tape at a reasonably constant speed, and within specific tension limitations. A further related object is the avoidance of undue stress to the magnetic tape at either spool.
Another object of the invention is the design of a transport system utilizing a pair of direct current motors to drive the takeup and supply spools. A related object is the achievement of a control circuit to provide a current at each motor according to the torque required at the corresponding spool.
A further object is the provision of transport control apparatus which utilizes the electrical characteristic of the drive motors to regulate the performance of the motors. A related object is the incorporation of electronic braking and torque control techniques.